Although muscle has some inherent regeneration capacity to repair damage resulting from physical or chemical trauma to muscle tissue, complete functional recovery of skeletal muscle after severe injury remains a challenge ^[1][2][52,53]. The return of muscular function is comprised by the incomplete recovery from muscle atrophy and fibrosis, which affects muscle fiber number, muscle cross-sectional area, and thus, muscle force ^[2][53]. 

While some in vitro works have shown direct formation of myotubes from AMSCs cultures ^[3][4][5][55,56,57], most investigations have shown that in vitro myogenic differentiation of AMSCs requires co-culture with myoblasts or satellite cells ^{[1][6][7][8][9][10][11][12]}[52,54,58,59,60,61,62,63]. Even though the in vitro differentiation potential of AMSCs into myocytes has been demonstrated, there is little evidence about AMSCs transplantation after skeletal muscle injury. Bacou et al. were the first to test the potential of AMSCs in a nonphysiological cardiotoxin induced muscle damage model in rabbits. Fifteen days after the transplantation, the cells expressed skeletal muscle markers, suggesting myogenic differentiation. In addition, two months after the treatment, muscles were heavier, showed a significantly larger fiber section area, and developed a significantly higher maximal force compared with damaged control muscles ^[13][43].

AMSCs promote the proliferation of myoblasts, which could explain the regenerative capacity shown in vivo ^{[14][12][15][16][17]}[30,63,64,65,66].

Muscular tears or lacerations are common lesions within athletes. Conventional therapies include medical management with non-steroideal anti-inflammatory drugs (NSAIDs), antioxidant therapy, and steroids; or surgical management such as myotenectomy or myectomy ^[18][19][67,69]. These therapies are unsuccessful and generally do not prevent the formation of fibrous tissue as well as do not promote muscle regeneration, leading to a limited function of the limb ^[18][20][67,68]. Therefore, regenerative therapies are gaining interest, particularly AMSCs, and some animal studies have been carried out in order to assess the in vivo efficacy of AMSCs in muscular tears. Peçanha et al. investigated how AMSCs contribute to skeletal muscle healing after a surgically performed laceration in the rat model. They conclude that AMSCs may accelerate the process of muscle repair, since the number of regenerating muscle fibers and muscle developed force significantly increased in the treated group ^[19][69]. Recently, Gorecka et al. have demonstrated that AMSCs transplantation into acute damaged skeletal muscle in a mice model, significantly improves functional muscle tissue regeneration without direct participation in muscle fiber formation ^[21][70].

RM is also gaining interest in the field of muscular dystrophies. Duchenne muscular dystrophy (DMD) is the most common and most severe form of muscular dystrophy. It is an X-linked genetic disorder caused by mutations in the dystrophin gene, which cause dystrophin deficiency. The loss of dystrophin leads to a breakdown of the structural integrity of myofibers, resulting in progressive myofibers necrosis, fibroblast proliferation, and growth of fibrous tissue and fat ^[22][71]. DMD is a progressive and lethal degenerating disease that affects both skeletal and cardiac muscle. Thus far, there is no effective treatment for DMD; however, several studies on cell therapy, including the application of MSCs have became a promising treatment to restore dystrophin in DMD patients. The use of AMSCs has also been proposed, but this therapy is still in preliminary testing and more experiments are required ^{[2][12][16][17]}[53,63,65,66]. Animal models have become increasingly important for testing these regenerative therapies. Mdx mouse is the most widely used animal model for DMD, presenting the same molecular and protein defect as seen in humans with the disease ^[23][72].

It was firstly observed that AMSCs treatment increases the content of VEGF and anti-inflammatory cytokines in dystrophin-deficient skeletal muscle, which promotes angiogenesis and reduces the inflammation, respectively. Moreover, a decrease in the content of TNF-α and Interleukin 6 (IL-6) were also reported in the AMSCs treated group, suggesting a protective action of the AMSCs on inflammation-induced injury. The AMSCs treated group showed improved muscle strength and resistance to acute muscle fatigue, after one injection of AMSCs per week during four weeks. In addition, a histological analysis was performed and an increase in fiber cross-sectional area and an augment of myogenin content was observed in AMSCs treated group ^[16][65]. Similar results were obtained by Lee et al., who demonstrated that AMSCs up-regulated myogenin, mTOR and raptor proteins, which contribute to the formation of myofibres. Thus, leads to an increase in muscle size when compared to control group ^[17][66]. These results support the proposition that AMSCs transplantation is a promising treatment for muscular dystrophies. Nevertheless, there is no clinical trials supporting these results; hence, further investigation is needed to determine the long-term effects of AMSCs in dystrophin-deficient muscles, and to find an accurate therapy for this pathology.

2. AMSCs in the Treatment of Tendon Injuries

The high incidence of tendon injuries is mainly associated with sport practice and aging, and they range from acute traumatic ruptures to chronic tendinopathy ^[24][25][73,74]. Tendon injuries represent a clinical challenge because their natural repair process is slow, complex, and inefficient, as well as a financial challenge. Tendon has limited inherent healing capacity, as it is a slightly cellular and poorly vascularized tissue, and often responds inadequately to treatments; hence, prolonged recovery times are needed ^[24][25][26][73,74,75]. After injuries, the structural composition and organization of tendons, which are responsible of the specific mechanical tendon properties, are not completely restored. Following the repair process, a fibrous scar is formed, causing significant dysfunction and joint movement inability, leading to a biomechanically weakened tendon, making it more vulnerable to re-rupture ^[25][27][74,76]. Tendinopathies and tendon tears have been treated with conservative approaches for managing symptoms, including rest, anti-inflammatory drugs, corticosteroids, and physiotherapy. On the other hand, the gold standard to treat tendon rupture is surgical suture, combined or not with allo- or auto-grafts ^[24][28][73,77]. Despite the improvements made in surgical techniques, none of the therapeutic options have provided successful long-term solutions ^[25][26][74,75]. New treatments are needed with the objective of improving tendon regeneration, and AMSCs have been adopted to repair tendon and ligament tears. It has been demonstrated that AMSCs can differentiate in vitro towards tenocytes. It is widely known that the control of stem cell activity is influenced by several environmental factors, including GF such as insulin like growth factor (IGF), TGF-β and growth differentiation factor 5 (GDF-5), which have been successfully used in a co-culture wit primary tenocytes, to promote AMSCs differentiation towards tenocytes in vitro ^[25][29][30][74,78,79]. AMSCs are also gaining interest among veterinarians. Race horses often suffer from superficial flexor digitorium longus tendon (SFDLT) lesions. There is clinical evidence that the injection of AMSCs after SFDLT spontaneous lesion significantly improves healing ^[31][87]. AMSCs were administered under ultrasonographic guidance in four horses suffering from SFDLT lesions. Treated horses showed shorter periods of lameness and better organization of collagen fibers assessed by ultrasound examination ^[31][87]. These results concur with those obtained by Carvalho et al., in a horse’s collagenase-induced SFDLT lesion controlled trial ^[32][88]. The histological evaluation demonstrated that AMSCs combined with a platelet concentrate therapy resulted in a better organization of collagen fibers and a decrease of the inflammatory infiltrate. In addition, the ultrasound evaluation showed a lack of lesion progression in the treated group ^[32][88] (Table 12).